A row has erupted over the rollout of a plan to allow secondary students who drop out of religious studies to attend a different class.

The head of a body representing community colleges warned they were already "pared back to the bone" and would need extra teachers to give students this option.

General secretary of Education and Training Boards Ireland Michael Moriarty was responding after Education Minister Richard Bruton said no extra resources would be poured into the plan to allow students to opt out of religion.

Mr Bruton suggested it was up to the schools to work it out as he said it was their "core mission" to respond to pupils' needs by reconfiguring their resources.

The new rules apply to 160,000 students attending almost half the country's secondary schools. They are State-run community colleges under the patronage of local education and training boards (ETBs) and community schools where a Catholic bishop or religious order shares patronage with one of these boards.

The rules do not apply to religious-run secondary schools that cater for more than half of post-primary students.

Parents have a constitutional right to withdraw their children from religious class, but schools have not been obliged to timetable them for another subject up to now. Many pupils who opted out would still have to sit through the class or study in the library.

Mr Moriarty said his organisation, which represents 274 schools and colleges, was inundated with calls from principals and directors of schools expressing concern about staff shortages yesterday.

"This will put a massive strain on stretched resources particularly in the context of significant shortages of teachers in science, maths, Irish and foreign languages," he said.

"The constant mantra is that you have to find the resources, but we're pared back to the bone over the last five years and are at the bottom of the barrel in terms of staffing resources.

"There aren't the resources to do this and it isn't going to be workable, particularly in urban areas where there is a high number of opt outs. What happens if subjects are already oversubscribed?"

He said people may underestimate the demand for opt outs as those in fifth or sixth year who are not taking religion as an exam subject may be tempted to drop out to avail of tuition in one of their exam options.

Mr Bruton said schools would "have to reconfigure their timetabling".

"We're not providing extra resources as part of this," he said. "This isn't a talk of developing a new programme, it's ensuring schools reconfigure the resources to meet the pupils' needs.

"You could extend the hours of some subjects, reducing the number of classes on the timetable on some subject areas depending on demand. This is the core mission of these schools, it's always been intended to respond to parents' needs."

The ASTI is seeking a meeting with the Department of Education about under-resourcing. Spokesperson Gemma Tuffy noted that its class size directive says they should range from 20 to 30 depending on the subject.

The TUI said it had concerns over the minister's "failure" to resource the plan. A spokesperson said other subject options must be provided during religion. "Quite clearly, this will require the employment of additional teachers," he said.